The present invention relates to electronic thermometers, and particularly to electronic thermometers having preheating devices to shorten the measuring time.
Thermistors have been widely used in electronic thermometer probes for measuring temperature. As described, for example, in U.S. Pat. No. 3,025,706, the thermistors generally used in such applications are those having a high negative temperature coefficient (NTC), i.e., having an electrical resistance which varies greatly in the negative direction with changes in temperature, as distinguished from ordinary resistors which have relatively small temperature coefficients. However, when using NTC thermistors for temperature measurement, long time intervals are generally required to make precise measurements because of the time needed for raising the temperature of the probe to that of the body whose temperature is being measured.
It is also known to shorten this time interval by preheating the probe to the approximate temperature of the body whose temperature is being measured. For example, U.S. Pat. No. 4,158,965 does this by applying auxiliary current to the thermistor to cause it to act as a heating element for raising the temperature of the probe, before the thermistor is used for making the precise temperature measurement. However, since the same thermistor is used for two functions, preheating and precise temperature measurement, its characteristics making it optimum for one function (e.g., preheating) may detrimentally affect its performance with respect to the other function (e.g., temperature measurement).